508
Climatic Geomorphology
and they can reach up to 9 m high and a density of 1000/ha. Large termitaria are built by
only a few species, while smaller termite-mounds are associated with numerous species.
Growing rates are very variable, from 25 to 500 mm/yr (Goudie, 1995). When termites
leave the termitaria, it is then exclusively submitted to erosive processes, leading to its
destruction. Although the termitaria material is much stronger than the underlying soil it
is finally eroded, normally in less than 10 years (Goudie, 1988). Termites also build small
filiform corridors upon the soil and vegetation, made up of cemented soil particles. These
accumulations give an idea of the amount of material involved in the removal process.
Following calculations made by several authors in Nigeria, Senegal, and Kenya, the
accumulation rate ranges between 300 and 1050 kg/ha/yr (Goudie, 1988).
Terrestrial worms also build constructions like termites although of lesser size, hardly
reaching 30 cm in height. Earthworms excavate burrows of up to 70 cm depth, which
indicates the importance of these organisms in the removal and mixing of soil components.
The amount of removed material is estimated as 5 to 25 mm/yr, greater than the quantity
moved by termites (Goudie, 1988).
All these weathering processes have a certain importance, but chemical weathering
plays the most relevant role in the disintegration of minerals and rocks in humid tropical
zones.
3.2. Chemical weathering
Chemical weathering results from a change in the chemical environment (Loughnan,
1969), because rocks formed in high pressure and temperature conditions and in the
absence of air become unstable when put in contact with the atmosphere. Water, oxygen,
and carbon dioxide attack the rock minerals through exothermic reactions. Most soluble
products are eliminated by dissolution, leaving a residue rich in less soluble components.
This residue is structured to produce neoformed compounds, mainly clay minerals and
iron and aluminium oxihydroxides or sesquioxides, which are in a more stable equilibrium
with the environmental conditions.
Jenny (1950) proposed a disintegration mechanism for the mineral structures. Following
the Pauling Rules, the sum of the positive charges in a crystal must be equal to the sum
of negative charges, but on the crystal surface valences are not compensated, resulting in
an unstable equilibrium. When crystals are in contact with water, the OH- ions join to
the existing cations and the H + ions with oxygen and other negative ions. The H + ion has
a small ionic radius and can occupy cation places in the crystal surface. Cations are then
released as hydroxides. In addition, the loss of H + ions modifies the pH of the solution.
3.2.1. Weathering of silicates
Silicates are the most common minerals in rocks. Approximate proportions of mineral
species in the Earth's surface are: feldspars (30 per cent), quartz (28 per cent), clay
minerals and micas (18 per cent), calcite and dolomite (9 per cent), iron oxides (4 per cent),
pyroxenes and amphiboles (1 per cent) and other minerals (10 per cent) (Leopold et al.,
1964). Cations released in the weathering reactions can be easily washed away or remain
in the residue. This is due to the physico-chemical variations of the solution. In this section
Climatic Geomorphology
and they can reach up to 9 m high and a density of 1000/ha. Large termitaria are built by
only a few species, while smaller termite-mounds are associated with numerous species.
Growing rates are very variable, from 25 to 500 mm/yr (Goudie, 1995). When termites
leave the termitaria, it is then exclusively submitted to erosive processes, leading to its
destruction. Although the termitaria material is much stronger than the underlying soil it
is finally eroded, normally in less than 10 years (Goudie, 1988). Termites also build small
filiform corridors upon the soil and vegetation, made up of cemented soil particles. These
accumulations give an idea of the amount of material involved in the removal process.
Following calculations made by several authors in Nigeria, Senegal, and Kenya, the
accumulation rate ranges between 300 and 1050 kg/ha/yr (Goudie, 1988).
Terrestrial worms also build constructions like termites although of lesser size, hardly
reaching 30 cm in height. Earthworms excavate burrows of up to 70 cm depth, which
indicates the importance of these organisms in the removal and mixing of soil components.
The amount of removed material is estimated as 5 to 25 mm/yr, greater than the quantity
moved by termites (Goudie, 1988).
All these weathering processes have a certain importance, but chemical weathering
plays the most relevant role in the disintegration of minerals and rocks in humid tropical
zones.
3.2. Chemical weathering
Chemical weathering results from a change in the chemical environment (Loughnan,
1969), because rocks formed in high pressure and temperature conditions and in the
absence of air become unstable when put in contact with the atmosphere. Water, oxygen,
and carbon dioxide attack the rock minerals through exothermic reactions. Most soluble
products are eliminated by dissolution, leaving a residue rich in less soluble components.
This residue is structured to produce neoformed compounds, mainly clay minerals and
iron and aluminium oxihydroxides or sesquioxides, which are in a more stable equilibrium
with the environmental conditions.
Jenny (1950) proposed a disintegration mechanism for the mineral structures. Following
the Pauling Rules, the sum of the positive charges in a crystal must be equal to the sum
of negative charges, but on the crystal surface valences are not compensated, resulting in
an unstable equilibrium. When crystals are in contact with water, the OH- ions join to
the existing cations and the H + ions with oxygen and other negative ions. The H + ion has
a small ionic radius and can occupy cation places in the crystal surface. Cations are then
released as hydroxides. In addition, the loss of H + ions modifies the pH of the solution.
3.2.1. Weathering of silicates
Silicates are the most common minerals in rocks. Approximate proportions of mineral
species in the Earth's surface are: feldspars (30 per cent), quartz (28 per cent), clay
minerals and micas (18 per cent), calcite and dolomite (9 per cent), iron oxides (4 per cent),
pyroxenes and amphiboles (1 per cent) and other minerals (10 per cent) (Leopold et al.,
1964). Cations released in the weathering reactions can be easily washed away or remain
in the residue. This is due to the physico-chemical variations of the solution. In this section